A conventional automatic umbrella as shown in FIGS. 4, 5 includes: a central shaft C consisting of plural telescopic tubes operatively extendible for opening the umbrella by an inner tension spring (not shown) held in the central shaft C and an outer tension spring S resiliently held in between an upper notch N fixed on a top end of the central shaft C and a middle runner M slidably held on the central shaft C; and a rib assembly R having a top linking rib R1 pivotally secured to the upper notch N, a top rib R2 pivotally secured to the middle runner M and the top linking rib R1, and a stretcher rib R3 pivotally secured to a lower runner L slidably held on the central shaft C.
Since there is no limiting or guiding mechanism provided for longitudinally guiding the sliding movement of the middle and lower runners M, L on the central shaft C, the rib assembly R as pivotally connected to the upper notch N, the middle and lower runners M, L may be twisted during the opening or closing operations of the umbrella, easily damaging the rib assembly and the umbrella structure.
The present inventor has found the drawbacks of the conventional automatic umbrella and invented the present automatic umbrella having anti-twisting property.